JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VI AUGUST 19, 1916 No. 14 



PHYSICS. — Recent improvements in the petrographic microscope. 

 F. E. Wright, Geophysical Laboratory. 



1. Sliding objective changer. In modern microscopes two forms 

 of objective changer are in common use. On ordinary, non- 

 polarizing microscopes (biological, etc.) the revolving nose piece 

 is universally favored and serves the purpose well. In case exact 

 centering is required, however, the revolving nose piece is me- 

 chanically inadequate and for this reason has never proved satis- 

 factory in petrographic microscope work. In its place some 

 form of objective clamp is usually adopted in polarizing micro- 

 scopes; but with such a clamp each change from one objective 

 to another involves a number of different operations which 

 together require from 10 to 30 seconds to accomplish; in the 

 course of a day's work, especially with fine grained and artificial 

 preparations, this may consume 5 and even 10 per cent of the 

 observer's available time. From an efficiency standpoint such 

 a procedure cannot be considered satisfactory; but it can fortu- 

 nately be remedied by a simple arrangement such that the time 

 involved in changing objectives is of the order of half a second 

 with the result that the total time is reduced to a fraction of 1 

 per cent of the day's working hours. The new device has been 

 in constant use for nearly a year and has proved its usefulness. 



The device is shown in figure 1 and consists simply of a sliding 

 brass carriage in which are mounted two objectives in excentric 

 conical steel rings, so cut that there is no change in focus in 

 passing from one objective to the second. Each objective is 



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